1. Field of the Invention
This invention relates to a mask structure for lithography, a method for preparation thereof and a lithographic method by use of the mask structure for lithography, and more particularly to a mask structure for lithography suitable for projecting a mask pattern on a wafer coated with a photosensitive agent for production of semiconductors, a method for preparation thereof and a lithographic method by use of said mask structure for lithography.
2. Description of the Prior Art
X-ray lithography provides a number of characteristics improving on lithography by visible light or UV-ray, such as the rectilinear propagation characteristic, incoherence, low diffraction characteristic, etc., and is attracting attention as a powerful means for submicron lithography.
While X-ray lithography has a number of superior points as compared with lithography by visible light or UV-ray, it involves drawbacks such as insufficient power of X-ray source, low sensitivity of resist, difficulty in alignment, difficulty in choice of mask material and working method, whereby productivity is low and cost is high, thus restricting its practical application.
Among them, referring now to the mask to be used for X-ray lithography, in lithography by visible light or UV-ray, glass plates and quartz plates have been utilized as the mask material holding member (namely light-transmitting member). However, in X-ray lithography, the wavelength of the ray which can be utilized is 1 to 200 .ANG.. The glass plates or quartz plates hitherto used substantially absorb the ray in this X-ray wavelength region, and as the thickness must be made as great as 1 to 2 mm, the X-ray cannot sufficiently be transmitted. These materials are therefore unsuitable as the material for the mask material holding member to be used for X-ray lithography.
X-ray transmittance generally depends on the density of a material, and therefore inorganic materials or organic materials with low density are being investigated as the material for the mask material holding member to be used for X-ray lithography. Such materials may include, for example, inorganic materials such as simple substances of beryllium (Be), titanium (Ti), silicon (Si) and boron (B), and compounds thereof, or organic compounds such as polyimide, polyamide, polyester, poly-p-xylylene, etc.
For practically using these substances as the material of the mask material holding member to be used for X-ray lithography, they are required to be made into thin films for maximizing the amount of X-ray transmitted, to a thickness of serveral microns or less in the case of an inorganic material and several tens of microns or less in the case of an organic material. For this reason, in forming, for example, a mask material holding member comprising a thin film of an inorganic material, or a composite film thereof, there is proposed the method in which a thin film of Si.sub.3 N.sub.4, SiO.sub.2, BN, SiC, etc. is formed by vapor deposition, etc., on a silicon wafer which is excellent in flatness, and thereafter the silicon wafer is removed by etching.
On the other hand, as the mask material (namely the X-ray absorbing material) to be used for X-ray lithography held on the mask material holding member as described above, it is preferable to use a thin film of a material having high density such as gold, platinum, tungsten, tantalum, copper, nickel, etc., preferably a thin film with a thickness of 0.5 to 1.mu.. Such a mask can be prepared by, for example, forming a thin film of the above high density material uniformly on the above mask material holding member, then applying a resist thereon, effecting a desired pattern drawing on the resist by electron beam, light, etc., and thereafter forming a desired pattern by means of etching, etc.
And, in the X-ray lithography of the prior art as described above, the X-ray transmittance through the mask material holding member is low, and the mask material holding member is required to be made considerably, thinner in order to obtain a sufficient amount of X-ray transmittance. Presentation of such a thin prior art mask material holding member is difficult.